This invention relates generally to digital data reproduction systems such as digital audio tape (DAT) recorders, and more particularly, to a method of, and means for, serially outputting the bits of data recreated in such apparatus for synchronization of the apparatus with some other apparatus, among other purposes.
In helical scan DAT recording, as in video recording, data is recorded on magnetic tape along a series of diagonal record tracks, as disclosed for example in European Patent Publication No. 356 938. Each track, corresponding to one frame of data, resolves itself a central PCM zone, a pair of automatic track finding (ATF) zones on both sides of the PCM zone, and a pair of subcode zones at the beginning and ending of the track. The PCM zones store the digitized audio signals, the ATF zones the tracking data, and the subcode zones the absolute time data which serves as the frame or track address.
It has recently been suggested that not only the absolute time data but additional data be recorded on the subcode zones for synchronous operation of DAT recorders and video recorders, that is, for synchronous playback of audio and video information. Currently, such recorders are of the professional class. The additional data includes time data representative of the time code specified by the International Electrotechnical Commission (IEC), and phase difference data representative of the phase difference between the DAT frames and the IEC time code frames. Each IEC time code frame is one thirtieth of a second long whereas each DAT frame is one thirty-third of a second long. Adaptation is therefore necessary for writing the IEC time code on the DAT frames.
A problem has been encountered in the synchronous operation of DAT recorders and video recorders by use of the IEC time code. The IEC time code frames are not necessarily supplied to DAT recorders, as well as to video recorders, with unvarying periodicity. Assume that the IEC time code frames input to the DAT recorder include ones that are not of the standard length. Then the IEC time code data created accordingly will be written on the subcode zones of the DAT frames. When the IEC time code frames are reconstructed by subsequently reading out the time code data from the DAT, they naturally include those of other than the standard length.
The IEC time code is comprised of, for example, 80 bits per frame. After recovery from the DAT, the bits of the IEC time code are serially delivered from a buffer memory, included in the DAT recorder, to the video recorder or the like for synchronizing data recovery therefrom with that from the DAT recorder. The buffer memory has so far been driven by clock pulses of a constant recurrence rate. For this very reason the IEC time code has not been correctly output in cases where the lengths of the reconstructed IEC time code frames deviate too much from the standard. Such incorrect IEC time code has seriously hampered the exact synchronization of DAT recorders and video recorders.
Similar problems have existed in synchronously operating two video recorders, a video recorder and an optical disk drive, and other combinations of various such known devices. In independent operation of such devices, too, fluctuations in a transfer rate of the bits of the recovered data with time are liable to occur due, for example, to variations in tape speed. Such fluctuations can make impossible the correct serial production of the data bits from a buffer memory being controlled by clock pulses of a constant recurrence rate.